BACTERIAL CONTAMINATION OF MILK AND CREAM.

New Zealand Journal of Agriculture, Volume XXVII, Issue 3, 20 September 1923, Page 179

BACTERIAL CONTAMINATION OF MILK AND CREAM.

HINTS FOR DAIRY-FARMERS

L. S. BARRELL,

Farm Dairy Instructor, Hokianga.

Milk as secreted in the udder of a healthy cow is germ-free. Contamination may take place in various ways : first, during the act of expressing the milk from the teat, as the opening from the milk-sinus to the outside is of such a nature that bacteria adhering to the mouth of the milk-duct are washed away with the first milk and therefore contaminate the whole milking. It will thus be seen that careful washing of the cow’s udder is the first step in the endeavour to produce a bacterially clean milk. Thereafter there are different sources of contamination that can, with thought and care, be eliminated to a large extent. The chief sources lie in the use of unclean dairy utensils and the air of the milking premises.

Too much stress cannot be laid upon the importance of thoroughly washing all dairy utensils, and finally scalding them with boiling water and retaining them in this boiling water for a few minutes.

Contamination from the air is effected by the multitude of fine particles of dust that are ever present in. the atmosphere of many

milking-sheds, each particle carrying its burden of living organisms which await more favourable surroundings, being unable to multiply until deposited in a medium containing the necessary moisture and food elements. These favourable conditions are found in milk, and the bacteria that thus find their way into the milk that is left standing about the dairy premises multiply with astonishing rapidity. Contamination by dust can to a large extent, be prevented by having concrete floors which are kept well washed down, and by the immediate separation of the milk and removal of the cream, as soon as separated, to a cool well-ventilated detached dairy.

Leaving aside the pathogenic or disease-causing bacteria, we may briefly consider that class of organism responsible for most of our defective home-separated creamnamely, the lactic-acid and particularly the gas-forming groups. Each of these types is represented by varieties or species, which may have different effects on the flavour, texture, or keeping-qualities of the dairy-product.

The gas-forming bacterium ■ gains access to the milk chiefly during the milking operation, through improper washing of the cow's udder or not washing the udder at all; dirty hands of the milker ; and most likely through the dirty habit of wet milking, when the milk is badly contaminated by the drippings from the hands. The organism becomes attached to the teats and . udder of the cow through contact with the earth when lying down, and particularly during the dry part, of the-season when cows are searching for water or succulent feed in swamps and about stagnant bog-holes. Its presence in cream is detected by fermentation, which usually begins to manifest itself within twenty-four hours, small bubbles of gas appearing on the surface. In bad cases it will be noted that the cream appears of a yeasty nature, and that its volume has visibly increased, due to displacement by the large volume of generated gases held in suspension. Cream when in this condition .is not suitable: for acceptance at the factory for manufacture into butter, as the fermentation is accompanied by a decidedly “ off ” flavour, and the aroma will vary with the particular species of gas-forming bacterium with which it is infected. Cream only slightly affected in this way has a flavour that the average dairyman. will put down to feed, and he will not concern himself about the matter further. As a matter of fact, however, it has been shown that in nine cases out of ten where it is assumed that feed-flavour is present the defect is directly due to the presence of gas-forming bacteria, and therefore controllable.

During the hottest weather of last summer a local dairy-farmer asked for the writer’s assistance in an endeavour to trace the . cause of a peculiar flavour in his cream that had kept his grade down when he was making every effort to produce a superfine article. This persistent flavour would . .have been commonly classed as having origin in the feed, but it was proved on investigation to be due to bacteria of the gas-forming type, although gas was not formed to such an extent as to show fermentation. It was found that the dairyman had his utensils, milking-machine, dairy, and shed in first-class condition as regards cleanliness. A curd test was made from a composite sample of the milk of the whole herd, and this showed after the usual incubation period that the milk was affected by gas-forming bacteria.

Thus the line of action was quite clear. The following day the milking-machine, , separator, and other utensils were thoroughly sterilized with, boiling water, and the writer took charge of the four subsequent milkings covering the period from which' the cream 'was’sent to the factory, personally washing and wiping dry each cow's udder, and making this operation more effective by the addition • of a small quantity of an odourless disinfectant to the washing-water. After each milking the machine plant, separator, cream-can, and other utensils were thoroughly washed and then sterilized with boiling water. Immediately after being separated the cream was removed from the separator-room to a dairy situated some distance from the shed, and therefore away from any likely source of contamination. Each skimming was kept separate, and stirred twice daily with a metal stirrer. Just before removal to the cream-stand to await transport to the factory the various skimmings were placed in a clean sterile can. From that day onward this farmer's cream has not failed to grade superfine, and has graded as high as 93.

Mention may be here made of a micro-organism that frequently causes trouble and was complained of by several dairy-farmers during last season —the one responsible for ropy or slimy milk and cream. Cream affected by this bacterium assumes a viscid or slimy nature, and its colour becomes much paler, which at once indicates to the trained eye that something is amiss. This trouble is not as a rule accompanied by any objectionable flavour, but rather by a lack of flavour— is, the cream does not possess' that full rich flavour found in the product that grades superfine. As cream thus affected will certainly not make first-class butter, it is either classed second grade or rejected altogether, depending on the extent of its infection. This organism is introduced into the milk chiefly by means of the washing-up water, particularly during the driest period of the season, when sources of water-supply frequently become stagnant and are mostly infected. For this latter reason also the germ can be introduced by improper washing of the cows' udders. Its presence in milk or cream can be prevented by the . same treatment as that mentioned in the case of gas-forming bacteria.

Finally, it is necessary to emphasize the importance of cooling for controlling the bacterial content of milk or cream. It is a well-known fact that those bacteria that form gas make their maximum increase at separating temperature — namely, between 8o° and 95 0 F. — while during very hot weather it is not unusual to find cream that has not been run over a cooler, with a temperature of over 8o° some considerable time after separation. With a well-designed cooler it is not difficult to get the temperature well below 70°, and in such circumstance gas-forming bacteria are outstripped in their race for existence by the less harmful lactic-acid germs, which eventually crowd out the former.

Acid formed in cream by the lactic-acid bacterium, unless present to such an extent that the acidity is very high, will not affect the grade of the product, providing it is possessed of a good clean flavour, fine texture, and colour, indicating that it has been well looked after and produced under sanitary conditions.